The present invention relates in general to attenuator circuits and, more particularly, to an attenuator circuit operating with a single point control voltage.
Attenuator circuits are commonly used in electronic circuit design, for example in communication systems such as portable telephones, to provide attenuation of a signal between two operating nodes. Attenuator circuits may be controlled with a digital signal, or with one or more analog control signals, to set the level of attenuation as needed in the particular application. In the minimum attenuation state, the attenuator circuit exhibits very low insertion loss to allow propagation of the incoming signal, say from a voltage controlled oscillator (VCO). Alternately, in the maximum attenuation state, the attenuator circuit exhibits high insertion loss to block the incoming signal.
The attenuator circuit should maintain a constant load impedance to the input node at minimum and maximum levels of attenuation. Maintaining a matched load impedance is important especially in portable telephone applications where it is necessary to avoid reflections which may cause frequency pulling on the VCO. Furthermore, to maintain simplicity of the design, it is desirable to control the attenuator circuit with a signal point control voltage.